Narrative:

During approach to fll airport on downwind leg on vectors for approach to 9L, I requested turn to final thinking that the controller would issue a visual approach. A clearance was issued to turn to a 130 degree heading to intercept the final approach course, and maintain 2500 ft until intercepting the localizer. During the turn to final the TCASII alerted us to a pop-up intruder. The target appeared to be almost under us, or over us, as it was yellow, and slightly to the left of our position. It was impossible to tell whether the aircraft was above or below as it's target was superimposed over the aircraft symbol. I stopped the turn on a 180 degree heading and leveled off at 3000 ft to avoid the target while attempting to locate the target visually. The other aircraft was never seen by either pilot, nor had the approach controller issued TA. After the conflict was over, I continued the turn to intercept the final approach course. The runway was in sight visually, and I began a descent for landing. The delay in the turn had taken us through the localizer, but visually we were very close to the final approach course. It seemed the normal thing to continue the descent for landing, but I had deviated from the published approach procedure. The controller then asked why we were at 1500 ft when we had been assigned 2500 ft ft until intercepting the localizer. In the distraction of the previously mentioned TCASII alert, I had just assumed that we had been cleared for a visual approach, when in fact, that clearance had been 'cleared for the approach,' not the visual approach. I realize that there is no room for assumptions when complying with ATC clrncs, but because my attention had been focused on the TCASII intruder, I had allowed my mind to think that what I had expected to happen had actually happened. In discussing this with my first officer, much to my chagrin, I realized that I had misunderstood the clearance.

Related reports:

Google
 

Original NASA ASRS Text

Title: MLG ON APCH VECTORS HAS TCASII RA, THEN DSNDS BELOW ASSIGNED ALT.

Narrative: DURING APCH TO FLL ARPT ON DOWNWIND LEG ON VECTORS FOR APCH TO 9L, I REQUESTED TURN TO FINAL THINKING THAT THE CTLR WOULD ISSUE A VISUAL APCH. A CLRNC WAS ISSUED TO TURN TO A 130 DEG HDG TO INTERCEPT THE FINAL APCH COURSE, AND MAINTAIN 2500 FT UNTIL INTERCEPTING THE LOC. DURING THE TURN TO FINAL THE TCASII ALERTED US TO A POP-UP INTRUDER. THE TARGET APPEARED TO BE ALMOST UNDER US, OR OVER US, AS IT WAS YELLOW, AND SLIGHTLY TO THE LEFT OF OUR POS. IT WAS IMPOSSIBLE TO TELL WHETHER THE ACFT WAS ABOVE OR BELOW AS IT'S TARGET WAS SUPERIMPOSED OVER THE ACFT SYMBOL. I STOPPED THE TURN ON A 180 DEG HDG AND LEVELED OFF AT 3000 FT TO AVOID THE TARGET WHILE ATTEMPTING TO LOCATE THE TARGET VISUALLY. THE OTHER ACFT WAS NEVER SEEN BY EITHER PLT, NOR HAD THE APCH CTLR ISSUED TA. AFTER THE CONFLICT WAS OVER, I CONTINUED THE TURN TO INTERCEPT THE FINAL APCH COURSE. THE RWY WAS IN SIGHT VISUALLY, AND I BEGAN A DSCNT FOR LNDG. THE DELAY IN THE TURN HAD TAKEN US THROUGH THE LOC, BUT VISUALLY WE WERE VERY CLOSE TO THE FINAL APCH COURSE. IT SEEMED THE NORMAL THING TO CONTINUE THE DSCNT FOR LNDG, BUT I HAD DEVIATED FROM THE PUBLISHED APCH PROC. THE CTLR THEN ASKED WHY WE WERE AT 1500 FT WHEN WE HAD BEEN ASSIGNED 2500 FT FT UNTIL INTERCEPTING THE LOC. IN THE DISTR OF THE PREVIOUSLY MENTIONED TCASII ALERT, I HAD JUST ASSUMED THAT WE HAD BEEN CLRED FOR A VISUAL APCH, WHEN IN FACT, THAT CLRNC HAD BEEN 'CLRED FOR THE APCH,' NOT THE VISUAL APCH. I REALIZE THAT THERE IS NO ROOM FOR ASSUMPTIONS WHEN COMPLYING WITH ATC CLRNCS, BUT BECAUSE MY ATTN HAD BEEN FOCUSED ON THE TCASII INTRUDER, I HAD ALLOWED MY MIND TO THINK THAT WHAT I HAD EXPECTED TO HAPPEN HAD ACTUALLY HAPPENED. IN DISCUSSING THIS WITH MY FO, MUCH TO MY CHAGRIN, I REALIZED THAT I HAD MISUNDERSTOOD THE CLRNC.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.